Fleas can develop resistance to certain chemicals. You might need to try a different brand. You might also consider treating your house so there are fewer fleas present that can bite your pets.
After having been bitten, some pets will develop a flea allergy. Then even only one flea bite will start the allergy cascade. DERMA-IONX can help with the allergy response. After you get rid of the fleas make sure you check a stool sample. Pets can acquire tapeworms after having had fleas. The tapeworms contribute to poor skin and coat. I would also give PROBIOTICS and OMEGA 3,6,9 beside DERMA-IONX to respond to the allergy.
Please let us know how you do.